The present invention relates to hypodermic syringes used to inject medication into human or animal tissue. In particular, the invention pertains to cartridge-type syringes having a chamber that accepts a pre-filled disposable ampoule cartridge containing medicant to be injected.
A reusable hypodermic syringe employing a disposable medicant cartridge and needle assembly is the predominant system used by dentists for the delivery of local anesthetics, e.g., carbocaine and octocaine. Such syringes are used for other injection procedures as well. The conventional system presents the caregiver with the problem of handling the distal end of the syringe to detach, dismantle and properly dispose of the needle assembly following the injection procedure.
A conventional cartridge-based syringe 1 is illustrated in FIG. 8. One such device is the ASTRA 9002-00 dental syringe, available from Gainor Medical Supply of McDonough, Ga. Syringe 1 comprises a main body 3 of stainless steel providing an elongated cylindrical chamber 5 for receiving through a front side aperture 6 a medicant filled fluid cartridge 7. On the rear side of body 3 is a smaller aperture 8 sized to allow the user to push with his/her finger a spent cartridge 7 back out of chamber 5, through aperture 6. Cartridge 7 has an elongated tubular body which is typically made of glass. One end of cartridge 7 is closed-off by an axially slidable elastomeric piston 9 providing a fluid-tight seal. The opposite end of cartridge 7 is sealed by a cap 11 having a centrally located pierceable diaphragm.
Fixedly attached to main body 3 is a contoured, generally spool-shaped finger grip 13. Grip 13 has a hollowed-out interior portion 15. Grip 13 also has a smaller axially centered passageway for slidably receiving an elongated plunger stem 17. Attached to a proximal end of plunger stem 17 is a plunger ring 19 designed to accommodate a user's thumb. Together, ring 19 and grip 13 allow precise finger control over the axial movement and position of plunger stem 17. Attached to the distal end of plunger stem 17 is a barbed plunger hook 21. Within interior portion 15 resides a coil spring 16, and a spring cup 18 attached to stem 17. These components serve to provide a slight bias of plunger hook 21 against piston 9 when cartridge 7 is positioned within chamber 5.
Threadably received on the distal end of main body 3 is a disposable needle assembly 23 comprising a needle hub 25, and a needle cannula 27 fixedly secured within hub 25 by a tight pressure fit provided, e.g., by a metal crimp ring 28. The proximal end of needle cannula 27 is pointed and protrudes from hub 25. When hub 25 is threaded onto the distal end of main body 3, needle cannula 27 extends into the lower part of chamber 5.
To use syringe 1, cartridge 7 is inserted into chamber 5, and the distal end of needle cannula 27 is caused to penetrate tissue at the desired injection location. Next, plunger stem 17 is advanced whereby cartridge 7 is pushed toward the distal end of chamber 5 and into contact with the protruding proximal end of needle cannula 27. Through application of additional finger pressure, the protruding proximal end of needle cannula 27 is caused to pierce cap 11 and enter cartridge 7. This pressure also causes the distal end of hook 21 to become embedded in piston 9. Thereafter, piston 9 advances within cartridge 7 to expel the medicant through needle cannula 27 and into the tissue.
The needle disposal method is less than ideal with conventional syringe 1. Typically, the entire needle assembly 23 is detached from main body 3 and disposed of in a special container. Since the needle is wholly exposed, it is also a good practice to cut or break-off the nee, die tip. This procedure is cumbersome and not always practical while working on a patient. As a result, the used needle may not be immediately properly disposed of. This significantly increases the risk of accidental needle strikes. Moreover, needles disposed of in this manner present a potential hazard to those that must handle removal of medical waste.
The potential for accidental needle strikes to health care givers and associated personnel poses a significant health risk. Needle strikes cause painful wounds, and can result in local infections or, worse yet, transmission of communicable diseases such as hepatitis. The problem has become particularly acute with the increasing treatment and care of those infected with HIV. At a minimum, a needle strike victim is likely to experience a high degree of anxiety over the possibility of adverse health consequences, and expensive testing for communicable diseases may be necessary.
The health risks associated with the conventional type of syringe extend beyond the medical community. Improper handling of medical waste can extend the risk of accidental needle strikes to children or others who may happen upon the waste, or lead to misuse of contaminated needles obtained by drug addicts rummaging through trash containers.
A syringe with a simple and reliable mechanism for providing needle retraction into a disposable medicant cartridge would contribute greatly to the reduction of the aforementioned health risks, and the spread of communicable diseases such as AIDS.
In conventional syringes of the type illustrated in FIG. 6, the embedding of barbed hook 21 within piston 9 allows retraction of piston 9 during the injection procedure. This is useful, e.g., when injecting local anesthetic, in order to check for blood flow indicating an improper placement of the needle end within a blood vessel. This retraction feature does not allow for retraction of a used needle into the disposable cartridge, however. First, no mechanism is provided for securely attaching the used needle cannula to the piston for retraction. Secondly, needle cannula 27 is held very securely in the needle bore, e.g., by metal crimp ring 28, and any embedded attachment of hook 21 and needle cannula 27 to piston 9 is bound to be too weak to withstand the large pulling force required to break the bond of the needle within the needle bore.
Previous attempts to address the potential of retractable needles in cartridge-type syringe systems have been unduly complex and of uncertain reliability. None is designed to allow for ready conversion of a conventional syringe into a safety syringe providing needle retraction into a disposable cartridge.
Haber et al. U.S. Pat. No. 4,808,169 discloses a "Disposable Safety Syringe Having Means for Retracting Its Needle Cannula Into Its Medication Cartridge." To enable piston retraction, this device provides a pair of gripping arms on a distal plunger end that is adapted to grip a projecting plug member provided on a proximal side of a slidable cartridge piston. At the end of a distal piston stroke, the proximal end of the needle becomes embedded in the piston. Also, the cartridge distal end is forced into contact with a pair of rotatable needle retaining jaws provided on a distal end of the cartridge receiving barrel. This contact rotates the jaws out of engagement with the needle freeing the needle to be retracted, with the piston, into the disposable cartridge.
Operational difficulties are apparent in the Haber et al. system. The system depends for its proper operation on a firm connection between the piston and the needle during retraction, yet such a connection cannot be ensured by simply embedding the needle cannula in the piston material. Moreover, the system requires that a distally directed force be continually exerted by the cartridge against the needle gripping jaws, to maintain the needle freely movable in the distal bore during needle retraction. This could be difficult given that needle retraction requires a proximal retraction of the piston that would tend to pull the cartridge out of contact with the needle gripping jaws.
Schroeder U.S. Pat. No. 4,813,936 similarly discloses an arrangement for retracting a hypodermic needle into a disposable anesthetic carpule (cartridge). According to this patent, the retraction is accomplished by the provision of a pointed distal plunger end that becomes embedded in a proximal side of a slidable plug (piston) of the carpule, and by a barbed or threaded proximal end of the hypodermic needle that becomes embedded in a distal side of the slidable plug at the end of the injection stroke.
A projection on the needle prevents the needle from moving in the distal direction, but apparently the needle is free to move in the proximal direction. Hence, once the plunger distal end and the needle proximal end become embedded in the slidable plug, the needle can be retracted from the needle bore and into the carpule.
Operational difficulties are apparent in this system as well. First, since no positive mechanism is evident for preventing proximal movement of the needle in the needle bore, proximally directed pushing forces, such as may be generated when the pointed distal end of the needle is pressed into tissue, may cause inadvertent proximal displacement of the needle. On the other hand, the disclosed system, wherein the plunger distal end and the needle proximal end are imbedded into the movable carpule plug, does not assure that the needle can be reliably withdrawn in the event that additional means are provided for firmly holding the needle in the needle bore.
Accordingly, there is a need for a simple and reliable cartridge-type syringe that provides sure retraction of a needle cannula into a disposable medicant cartridge. Ideally, such a system would be readily adaptable for use with existing standard cartridge-type syringes, e.g., of the type illustrated in FIG. 8.